The invention relates to a centering device for a mechanical tracer, and more particularly, to a centering device for a mechanical tracer having a driver and at least two pairs of return means acting on the latter in opposite directions.
A centering device is disclosed in German Patent Specification 2,356,030, in which there is provided for the coordinate measurement of a workpiece a tracer which is arranged on a spring parallelogram. When it runs along the workpiece, the tracer can drift by means of the flexion of a flexural bar and as a result of the suspension on the spring parallelogram. In order to return the tracer to its exact initial position after each measurement, a centering device is provided. In this centering device, resilient means are additionally provided in the case of each spring parallelogram in order to vary the force/path characteristic. The aim of this design is to limit the force acting on the tracer to an approximately constant maximum force even when pronounced deflections occur. The driver, at its end, carries a ball, on which springs of equal force and low spring constant act on both sides via plane thrust pieces. An arrangement of three balls in the centering device acts respectively on the two thrust pieces independently as a thrust and as a second stop for the first pieces loaded by the springs.
In order to guarantee a perfect hysteresis-free centering in the basic position, all the contact points provided between the ball of the driver, the balls of the stops and the thrust pieces must be in exact bearing contact at the same time. This is possible only at a high production outlay in terms of the accuracy of the balls and of the plane thrust pieces. During use, this quality necessarily diminishes as a result of wear, for example, by impact effect or by the friction of the ball of the driver on the thrust pieces.
The centering device according to German Offenlegungsschrift 4,027,136 constitutes a further development. In this centering device, the return means acting in opposite directions are designed in such a way that a stop, which can be multi-part, is provided for one return means. The return means bearing against the stop in the state of rest exerts, at least near the state of rest, twice as high a return force as the second opposite return means. Because the return means bearing against the stop does not act on the driver when the driver is deflected counter to the return force of the opposite return means, only the single return force acts on the ball of the driver. If the driver is deflected in the other direction, although twice as high a return force acts on the ball of the driver, nevertheless this return force counteracts the single return force of the opposite return means. Thus, in actuality, only the difference of the two return forces takes effect. A return force of equal amount is thereby obtained in both directions. Either spring-loaded pistons or pistons loaded by means of a fluid are provided as return means.
This design makes it possible, at most, to vary the maximum force occurring during deflection by varying the system pressure, but not to alter the region of the characteristic which is linear about the zero point. If the tracer system is to be matched as closely as possible to different measuring tasks, different centering devices have to be used for these different measuring tasks, and for each special measuring task it is necessary to use a special centering device which has the optimum force/path characteristic for the particular measuring task.
Furthermore, these centering devices belonging to the state of the art are sensitive to the vibrations of the tracer which are excited, for example, when the measuring head is moved back and forth between different measuring positions. These vibrations are dependent on the accelerations of the tracer. The vibrations disturb the measurement considerably when the tracer runs along the workpiece, and therefore the tracer has to be run onto the workpiece without appreciable acceleration, this being undesirable in terms of process technique.